In "The Bully," he is revealed to either be the manager or an employee of Pizza Castle.
In "Suction Cup Symphony," he is revealed to either be the manager of Pizza Castle or his own restaurant, hence how SpongeBob remarks how Squidward might have ate at Mario's the previous night.
Mario is mentioned when Patrick asks for him when he accidentally calls Mrs. Puff's Boating School, mistaking the school for Pizza Castle. His name and him working at the aforementioned restaurant both are mentioned in this episode.
"Suction Cup Symphony"
Thinking that Squidward's clarinet playing is him flatulating, SpongeBob is given the impression that Squidward ate at Mario's restaurant the preceding night. This gives a small hint that Mario's pizza might be known to give one gas.
- At some point, he was either an employee or the manager of Pizza Castle.
- At some point, he either became the manager of Pizza Castle or his own restaurant.